It is well recognized that the operation of a television picture tube, one form of a cathode ray tube, involves the emission of a beam of electrons from a cathode. The conventional television receiver tube consists of three principal parts: an electron emitting means, an envelope for containing the electron emitting means, and a faceplate through which the picture is viewed, which is attached to the envelope (also referred to as the funnel). The beam of electrons moves over a screen on the faceplate to selectively activate points or dots of phosphor applied to the inside surface of the faceplate to thereby create an image. This bombardment of the phosphor by the stream of electrons causes the production of X-radiation, an undesirable side effect.
The safety of the picture viewer dictates that the electrons and X-rays be confined within the television tube. The faceplate portion of the tube presents a problem since it must remain clear and transparent to the viewer. The problem has intensified in recent years as higher voltages have been used to generate the electrons. Present government regulations restrict the transmittance of X-ray radiation to not more than 0.5 milliroentgen per hour (mr/hr) measured at 5 cm from the faceplate. Accordingly, research has been constant to devise glass compositions for faceplates which provide the necessary absorption of X-radiation while retaining the desired visible transmission therethrough.
Lead oxide (PbO) is well-known for its capability of absorbing X-radiation. Unfortunately, however, electron bombardment of glasses containing substantial quantities of PbO gives rise to a serious discoloration which has been termed "browning". Such discoloration is not only unpleasing aesthetically but it also reduces the visible transmission of the faceplate and, in the case of color television, deleteriously affects the color of the picture.
It has been appreciated that this browning may be the result of either X-radiation or electron impingement on the glass, or from both actions. U.S. Pat. No. 2,477,329 describes the addition of cerium oxide (CeO.sub.2) to faceplate glass compositions to inhibit X-radiation discoloration and the essential elimination of easily reducible oxides from the compositions, referring explicitly to lead oxide, to minimize electron browning. The patent teaches the substitution of barium oxide (BaO) for PbO. British Pat. No. 870,101 discloses the beneficial effect which the combination of CeO.sub.2 and titanium dioxide (TiO.sub.2) has upon inhibiting X-ray browning. The addition of TiO.sub.2 permits the amount of CeO.sub.2 to be minimized, thereby reducing cost.
U.S. Pat. No. 2,527,693 discloses glasses having compositions within the alkali metal oxide-barium oxide-aluminum oxide-silica (R.sub.2 O-BaO-Al.sub.2 O.sub.3 -SiO.sub.2) system which have formed the basis for commerically-used television tube faceplates. Nevertheless, as the tube voltages have increased, glasses exhibiting greater capacity for absorbing X-radiation and greater resistance to electron and X-ray browning have been sought. One proposal for solving the problem has involved utilizing larger amounts of BaO in the glass composition (British Pat. No. 1,123,857 and British Pat. No. 1,231,378). Unfortunately, when sufficient BaO was included to satisfy the X-ray absorption requirements, glass melting problems developed and the liquidus temperature was raised sharply.
U.S. Pat. No. 3,464,932 describes glasses in the R.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 field containing strontium oxide (SrO). SrO exhibits a greater capacity for absorbing X-radiation having wavelengths within the range of 0.35-0.77 A than does BaO. SrO, like BaO, is resistant to electron browning.
However, with the advent of television projection systems, i.e., the picture is projected onto a large screen spaced away from the television tube, still higher operating voltages have been damanded with consequent greater X-ray and electron emissions. Thus, the present glasses utilized in the manufacture of television tube faceplates in the United States rely upon SrO as the primary X-ray absorber. X-radiation absorption is limited to a linear X-ray absorption coefficient of about 25 cm.sup.-1 at a wavelength of 0.6 A which corresponds to a SrO content of approximately 14% by weight SrO. Concentrations of SrO in excess of about 14% hazard crystallization of strontium silicate as the molten glass is cooled. This situation has led to the need for a second glass faceplate which absorbs radiation passing through the first faceplate or the use of a much thicker-walled single faceplate. Both of those alternative solutions to the problem are unattractive commercially since they require redesign of the projection unit.
Total X-ray absorption comprises the product of faceplate thickness and linear absorption coefficient. It has been determined that satisfactory X-ray absorption with faceplates of the same thickness as the present commercial faceplates could be achieved with glasses having linear X-ray absorption coefficients at 0.6 A of at least 35 cm.sup.-1 and, preferably, in the vicinity of 40 cm.sup.-1. Such glasses insure that X-ray transmittance will be less than 0.5 mr/hr when measured at a distance of 5 cm from a faceplate of conventional thickness.